Electric power providers owning nuclear power plants have stored a massive amount of used nuclear fuel, and establishment of the method for safely and effectively processing such used nuclear fuel has been an urgent issue.
For this reason, study has been conducted on a nuclear fuel cycle that fissionable U-235 or Pu is extracted from the used nuclear fuel and about 3 to 5% of the resultant is mixed with non-fissionable U-238 to reproduce new fuel.
A used nuclear fuel of about 20 tons is annually produced or yielded from a 1000 MWe class nuclear power plant. Used nuclear fuel of 3%-enriched uranium fuel (U-235: 3%, U-238: 97%) contains 1% of U-235, 95% of U-238, 1% of Pu, and 3% of other products. These products are categorized into minor actinide (MA), platinum groups, short-lived fission products (SLFP), and long-lived fission products (LLFP).
Note that these products exhibit high neutron absorbing properties, and are the cause of interfering with progress of chain reaction of nuclear fission along with increasing of those amounts.
For this reason, these products are much contained in highly active liquid waste (HALW) inevitably caused by reprocessing of the used nuclear fuel and vitrified waste in such a form that the highly active liquid waste can be discarded.
When this highly active liquid waste (HALW) is, without change, formed into the vitrified waste for disposal, a massive amount of high-level radioactive waste generating heat needs to be managed for thousands of years, leading to a burden increase. Actually, the vitrified waste has been already held, and therefore, long-term management has been demanded.
For these reasons, for the purpose of reducing a burden due to disposal of the highly active liquid waste (HALW) and management of the already-held vitrified waste, study has been conducted on the technique of separating contained nuclides into groups according to a half-life or chemical properties and selecting, for each group, a disposal method according to properties. Thus, a storage period of the high-level radioactive waste can be shortened, and a storage space can be further saved.
For the groups with the long-lived fission products (LLFP) among the groups separated from the highly active liquid waste (HALW) and the vitrified waste, study has been conducted on application of the technique of nuclear transmutation into short-lived radionuclides or stable nuclides.
Specifically, the technique of nuclear transmutation into isotopes with a shorter half-life by application of photonuclear reaction (γ, n) for irradiating the long-lived fission products (LLFP) with a gamma beam to cause neutron emission or neutron capture reaction (n, γ) for irradiating the long-lived fission products (LLFP) with neutrons to cause gamma beam emission has been disclosed (e.g., Patent Literatures 1 and 2).